1. Field of the Invention
This invention relates generally to compensation of chromatic dispersion, including for example compensation of dispersion slope. More specifically, this invention relates to the use of etalons to compensate for chromatic dispersion.
2. Description of the Related Art
As the result of recent advances in technology and an ever-increasing demand for communications bandwidth, there is increasing interest in optical communications systems, especially fiber optic communications systems. This is because optical fiber is a transmission medium that is well suited to meet the demand for bandwidth. Optical fiber has a bandwidth, which is inherently broader than its electrical counterparts. At the same time, advances in technology have increased the performance, increased the reliability and reduced the cost of the components used in fiber optic systems. In addition, there is a growing installed base of laid fiber and infrastructure to support and service the fiber.
However, even fiber optic systems have limits on price and performance. Chromatic dispersion is one basic phenomenon, which limits the performance of optical fibers. The speed of a photon traveling along an optical fiber depends on the index of refraction of the fiber. Because the index of refraction is slightly dependent on the frequency of light, photons of different frequencies propagate at different speeds. This effect is commonly known as chromatic dispersion. Chromatic dispersion causes optical signal pulses to broaden in the time domain. In addition, chromatic dispersion is cumulative in nature. Therefore, optical signals, which travel longer distances, will experience more chromatic dispersion. This limits the signal transmission distance over which high bit rate signals can be transmitted, even with the use of narrow linewidth lasers and low chirp external modulators. For instance, signals at 10 Gbps can travel roughly 80 km in a standard SMF-28 single mode fiber before adjacent digital bits start to interfere with each other. At 40 Gbps, this distance is reduced to 6 km. Chromatic dispersion is a significant problem in implementing high-speed optical networks.
Several different approaches have been proposed to compensate for the effects of chromatic dispersion and, therefore, extend the signal transmission distance. They include systems based on dispersion compensating fiber, fiber Bragg gratings, photonic integrated circuits and etalons.
Dispersion compensating fibers (DCF) are optical fibers which have chromatic dispersion which is opposite in sign to the chromatic dispersion in “normal” fibers. Thus, propagation through a length of DCF cancels the chromatic dispersion, which results from propagating through standard single mode fiber. At the present time, DCF is one of the leading commercial technologies for the compensation of chromatic dispersion and a significant number of chromatic dispersion compensating devices is based on DCF. However, DCF has several significant disadvantages. First, long lengths of DCF are required to compensate for standard fiber. For example, a typical application might require 1 km of DCF for every 5 km of standard fiber. Thus, 100 km of standard fiber would require 20 km of DCF. These amounts of DCF are both expensive and bulky. Second, DCF solutions are static. A 20 km length of DCF will introduce a specific amount of dispersion compensation. If more or less is required, for example due to changes in the overall network architecture, a different DCF solution must be engineered. The existing 20 km of DCF cannot be easily “tuned” to realize a different amount of dispersion compensation, making it unsuitable for agile telecommunications network applications. Third, DCF is a type of fiber and suffers from many undesirable fiber characteristics, typically including undesirable fiber nonlinearities and high losses. A 20 km length of fiber can introduce significant losses. Fourth, standard single mode fibers have non-uniform dispersion values over a wide bandwidth, resulting in a second-order dispersion effect commonly referred to as dispersion slope. DCF solutions typically do not compensate for dispersion slope, leaving behind some uncompensated residual dispersion.
Fiber Bragg gratings (FBG) have emerged over the past few years as a promising candidate for the compensation of chromatic dispersion. A fiber Bragg grating is a length of fiber into which Bragg gratings have been formed. Various groups have proposed different architectures for using FBGs to compensate for chromatic dispersion. For example, see FIG. 1 in C. K. Madsen and G. Lenz, “Optical all-pass filters for phase response design with applications for dispersion compensation,” IEEE Photonics Technology Letters, vol. 10, no. 7, July 1998, pp. 994–996. However, practical implementation of FBG solutions remains difficult. Engineering limitations have resulted in less than acceptable dispersion compensation. Finding reproducible and reliable processes to make a dispersion compensator based on FBGs remains very challenging. In addition, Bragg gratings are inherently narrow band devices so FBG-based dispersion compensators typically have a narrow operating bandwidth. It is also difficult to tune FBGs to achieve different amounts of dispersion compensation.
Architectures based on planar waveguides have also been proposed. For example, the paper referenced above suggests an approach for compensating for chromatic dispersion using an all-pass filter approach based on ring structures in planar waveguides. However, this approach is inherently expensive and polarization sensitive.
Finally, around 1990, it was disclosed that the phase response of a single etalon has a nonlinear relationship with frequency. See L. J. Cimini Jr., L. J. Greenstein and A. A. M. Saleh, “Optical equalization to combat the effects of laser chirp and fiber dispersion,” J. Lightwave Technology, vol. 8, no. 5, May 1990, pp. 649–659. Furthermore, it was proposed that an etalon could be used to compensate for chromatic dispersion. Since that time, various etalon-based architectures have been suggested. However, most, if not all, of these architectures suffer from significant drawbacks. Many of them simply cannot attain the necessary performance. They often suffer from too much group delay ripple (e.g., >20 ps) and/or too narrow an operating bandwidth. In addition, most, if not all, designs are static. The designs cannot be easily tuned to achieve different amounts of dispersion compensation. In addition, they typically do not adequately compensate for dispersion slope.
Thus, there is a need for dispersion compensation systems, which can be tuned to achieve different amounts of dispersion compensation, including different amounts of dispersion slope compensation for some applications. It is also desirable for these systems to operate over a large bandwidth and to be capable of achieving low group delay ripple.